I have read that when bosons interfere they do so by adding the probability amplitudes, then I read that when fermions interfere they do so by subtracting the probability amplitudes.

The usual double slit interference pattern has a bright fringe in the middle and weaker fringes around it. If electrons (with same spin) are used and they interfere by subtracting the amplitudes, then we would get a negative interference pattern with a dark fringe in the middle and two fringes of equal strength around it don't we?